If you ask the prime minister what he thinks of PMQs, he will always give the same answer: he shakes his head and tells you he doesn't enjoy it because it's childish. It is the politik thing to say when you're at a reception in No 10. When you're in the bearpit of the Commons chamber, the politik thing to do is to savage the opposition.

Such is the education provided by Eton that it has furnished David Cameron with the ability to be as adept in the cultivated atmosphere of Downing Street soirees as he is in the schoolboy fug of the House of Commons. The prime minister expects no quarter and gives none. The reason he wins more of these exchanges than he loses is because he is a better bully than Ed Miliband.

It was towards the end of today's half-hour that this point was underlined in brutal playground fashion. Miliband had long since used up his six questions, meaning he had no further chances to have another go at the PM. It was the perfect time for a bully to strike. His cronies, those toadying government backbenchers happy to help their leader with a planted question, had raised the question of the controversial benefit cap. This was an issue on which Labour's position is far from clear.

"Will he back us in the voting lobby tonight?" Cameron asked Miliband. "Just nod," the PM mocked, staring across the despatch box. Miliband had no response. He just stared straight back, like a teenager playing poker, his dulled face utterly impassive. The leader of the opposition looked like a stuffed tortoise. Only tortoises get to retreat into their shells, don't they? For Miliband, there was nowhere to hide.

Tory MPs cheered and jeered their leader, urging him on to kick this helpless little second-year again and again. Cameron repeated the trick twice more. "Absolutely hopeless!" he taunted. Miliband gave the impression of being about to burst into tears. "One more go? Nothing!" The Labour leader could not even muster the presence of mind to come up with a retort of his own.

This is not the first time Cameron has tried these tactics. When he used them against Gordon Brown the ex-PM would suddenly develop an enthusiasm for entering into intensive conversations with his chancellor Alistair Darling. Miliband did not even have that option, as Ed Balls was busy flapping his arms around to demonstrate, rather perversely, his own unflappability. Miliband looked like a turtle out of water. Slow-moving. Vulnerable. Likely to be turned into turtle soup.

That moment, as Cameron towered over him jabbing his finger, seemed to sum up the state of Miliband's opposition in early 2012. The occasional purple patch in which he has allowed exuberance to lift his performance is keeping Miliband in the job, but overall he is sluggish in response to events - shaping the agenda, but unable to think on his feet. Today he was sitting down when the barrage of bullying came, which probably didn't help.

Miliband should have seen it coming. Earlier Cameron had been rebuked by the Speaker, John Bercow, for criticising the "hypocrisy" of Miliband's opposition to the government's executive pay. This unparliamentary language was just not the done thing, the Speaker explained, as he told Cameron to withdraw the remark.

"I'm very happy to do that, Mr Speaker," the PM replied in a wheedling voice. There then followed a lengthy explanation of why it was that Miliband was, in his view, hypocritical. Labour MPs gnashed their teeth at this blatant side-stepping.

The Speaker's intervention came after Miliband gave a fighting response of his own to Cameron's jibe. "I'll tell you what hypocrisy is!" he said quickly, getting a cheer as he raised the government first letting Stephen Hester's bonus go through and then approving its removal.

For some reason the Speaker did not call on Miliband's "hypocrisy" riposte to be withdrawn. How odd. We can only conclude that he was working on the inexorable logic of the playground: 'he started it'.